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  3. The nuclear polyhedrosis virus helps control gypsy moth populations by killing the moth’s larvae. The virus is always present in

The nuclear polyhedrosis virus helps control gypsy moth populations by killing the moth’s larvae. The virus is always present in

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问题 The nuclear polyhedrosis virus helps control gypsy moth populations by killing the moth’s larvae. The virus is always present in the larvae, but only every sixth or seventh year does the virus seriously decimate the number of larvae, thereby drastically setting back the gypsy moth population. Scientists believe that the virus, ordinarily latent, is triggered only when the larvae experience biological stress. If the scientists mentioned above are correct, it can be inferred that the decimation of gypsy moth larvae populations by the nuclear polyhedrosis virus would be most likely to be triggered by which of the following conditions?
选项 A、A shift from drought conditions to normal precipitation in areas infested by gypsy moths.
B、The escalating stress of defoliation sustained by trees attacked by gypsy moths for the second consecutive year.
C、Predation on larvae of all kinds by parasitic wasps and flies.
D、Starvation of the gypsy moth larvae as a result of overpopulation.
E、Spraying of areas infested by gypsy moths with laboratory-raised nuclear polyhedrosis virus.
答案D
解析 本题重点有两个：(1)明晰问题目的，当问题目的较长时尤应如此；(2)读题重点应放在段落最后一句话“only when the larvae experience biological stress”。(D)中的“overpopulation”与“biologicalstress”相吻合，所以(D)正确；(A)起到了驳斥作用；(B)中的“trees”为新概念；(C)中的“all kinds”把范围扩大化了；(E)也与段落推理无关。
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